Refrigerating machine oil composition for carbon dioxide refrigerant

ABSTRACT

The invention relates to a refrigerator oil composition for compression refrigerators with carbon dioxide refrigerant therein, such as car air conditioners, refrigerators, freezers, air conditioners, heat pumps, etc. The base oil of the composition comprises an oxygen-containing organic compound as the essential component and has a kinematic viscosity at 100° C. of from 5 to 50 mm 2 /sec, a viscosity index of at least 60, and a water content of at most 300 ppm.

TECHNICAL FIELD

The present invention relates to a refrigerator oil composition forcarbon dioxide (CO₂) refrigerant. The refrigerator oil composition canbe used in compression refrigerators with carbon dioxide refrigeranttherein, concretely in refrigerating or heating-cooling equipment suchas car air conditioners, refrigerators, freezers, air conditioners, heatpumps, etc.

BACKGROUND ART

In general, a compression refrigeration cycle for refrigerators, forexample, that for compression refrigerators comprising a compressor, acondenser, an expansion valve and an evaporator is of a closed system inwhich is circulated a mixed liquid comprising a refrigerant and alubricating oil. In compression refrigerators of that type, in general,chlorofluorocarbons such as dichlorodifluoromethane (R-12) andchlorodifluoromethane (R-22) have heretofore been used as refrigerants.Various types of lubricating oils have been produced and used togetherwith such refrigerants. However, when released in air, these Floncompounds that have heretofore been used as refrigerants will bringabout environmental pollution, as destroying the ozone layer existing inthe stratosphere. Therefore, their use is being severely controlled inall the world. Given the situation, new refrigerants, hydrofluorocarbonsand fluorocarbons such as typically 1,1,1,2-tetrafluoroethane (R-134a)have become specifically noted. Hydrofluorocarbons and fluorocarbonswill not destroy the ozone layer. However, as their life in air is long,they will cause global warming. Therefore, the recent tendency in theart is toward using refrigerants from natural resources that are freefrom the problem.

Carbon dioxide is excellent as it is harmless to the environment and issafe to human beings. In addition, it has the advantages of anywhereeasy availability and extremely low costs with no necessity forrecovery. For these reasons, carbon dioxide has been much used as therefrigerant for refrigerators.

However, using carbon dioxide as a refrigerant is often problematic inthat the system with carbon dioxide therein requires higher jettingpressure and has higher temperature, as compared with that where is usedR-134a or the like, with the result that the refrigerator oil in thesystem is exposed to carbon dioxide of being in a supercriticalcondition. Therefore, CO₂-based refrigerators, if lubricated withordinary lubricating oil, involve unexpected problems in that thelubricating oil used therein is degraded and the machine parts arecorroded and undergo copper plating, and, after all, the refrigeratorscould not be stably driven for a long.

The present invention has been made in consideration of the viewpointsnoted above, and its object is to provide a refrigerator oil compositionwhich is not degraded, even when used in CO₂-based refrigerators,neither corroding the machine parts nor causing copper plating, andtherefore ensures stable long-term driving of CO₂-based refrigerators.

DISCLOSURE OF THE INVENTION

We, the present inventors have assiduously studied and, as a result,have found that the object of the invention can be effectively attainedby using a composition comprising a specific base oil of which the watercontent is defined not to overstep a predetermined level, or by usingsuch a composition containing a specific additive. On the basis of thesefindings, we have completed the present invention. Specifically, theinvention provides the following:

[1] A refrigerator oil composition for carbon dioxide refrigerant, ofwhich the base oil comprises an oxygen-containing organic compound asthe essential component and has a kinematic viscosity at 100° C. of from5 to 50 mm²/sec, a viscosity index of at least 60, and a water contentof at most 300 ppm.

[2] The refrigerator oil composition for carbon dioxide refrigerant ofabove [1], of which the water content of the base oil is at most 250ppm.

[3] The refrigerator oil composition for carbon dioxide refrigerant ofabove [1] or [2], wherein the oxygen-containing organic compound is oneor more selected from polyoxyalkylene glycols, polyvinyl ethers,polyesters and carbonates.

[4] The refrigerator oil composition for carbon dioxide refrigerant ofabove [3], wherein the polyoxyalkylene glycols are represented by thefollowing general formula (I) and the polyvinyl ethers are by thefollowing formula (V):R¹—[(OR²)_(m)—OR³]_(n)   (I)where R¹ represents a hydrogen atom, an alkyl group having from 1 to 10carbon atoms, an acyl group having from 2 to 10 carbon atoms, or analiphatic hydrocarbon group having from 2 to 6 bonding sites and havingfrom 1 to 10 carbon atoms; R² represents an alkylene group having from 2to 4 carbon atoms; R³ represents a hydrogen atom, an alkyl group havingfrom 1 to 10 carbon atoms, or an acyl group having from 2 to 10 carbonatom; n represents an integer of from 1 to 6; and m represents a numberto give a mean value of m×n falling between 6 and 80;

where R¹⁶, R¹⁷ and R¹⁸ each represent a hydrogen atom, or a hydrocarbongroup having from 1 to 8 carbon atoms, and they may be the same ordifferent; R¹⁹ represents a divalent hydrocarbon group having from 1 to10 carbon atoms; R²⁰ represents a hydrocarbon group having from 1 to 20carbon atoms; k represents a number of from 0 to 10 on average; R¹⁶ toR²⁰ may be the same or different in different structural units; andplural R¹⁹O's, if any, may be the same or different.

[5] The refrigerator oil composition for carbon dioxide refrigerant ofabove [3] or [4], wherein the polyesters are polyalcohol esters orpolycarboxylates.

[6] The refrigerator oil composition for carbon dioxide refrigerant ofany of above [1] to [5], which contains an acid scavenger.

[7] The refrigerator oil composition for carbon dioxide refrigerant ofabove [6], wherein the acid scavenger is one or more selected fromphenyl glycidyl ethers, alkyl glycidyl ethers, alkylene glycol glycidylethers, cyclohexene oxides, and α-olefin oxides.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart showing one embodiment of a compressionrefrigeration cycle in which is accepted the refrigerator oilcomposition of the invention.

FIG. 2 is a flowchart showing another embodiment of a compressionrefrigeration cycle in which is accepted the refrigerator oilcomposition of the invention.

FIG. 3 is a flowchart showing still another embodiment of a compressionrefrigeration cycle in which is accepted the refrigerator oilcomposition of the invention.

FIG. 4 is a flowchart showing still another embodiment of a compressionrefrigeration cycle in which is accepted the refrigerator oilcomposition of the invention.

In these, 1 indicates a compressor; 2 indicates a condenser; 3 indicatesan expansion valve; 4 indicates an evaporator; 5 indicates an oilseparator; 6 indicates a hot gas line; and 7 indicates a hot gas linevalve.

BEST MODES OF CARRYING OUT THE INVENTION

Embodiments of the invention are described hereinunder.

The base oil for use in the invention comprises, as the essentialcomponent, an oxygen-containing organic compound and has a water contentof at most 300 ppm, preferably at most 250 ppm, more preferably at most200 ppm. Base oil having a water content of larger than 300 ppm willincrease the acid component of the refrigerator oil comprising it tothereby promote the degradation of the refrigerator oil and, inaddition, it will corrode metallic machine parts and cause copperplating to thereby interfere with the performance of refrigerators. Toreduce their water content, for example, oxygen-containing organiccompounds such as those mentioned below may be heated in vacuum or maybe contacted with silica gel, activated alumina, zeolite or the like.Preferably, they are contacted with zeolite (for example, naturalzeolite, or synthetic zeolite such as Molecular Sieves 3A, 4A, 5A, 13X)having a particle size of from 0.1 to 10 nm, more preferably from 0.2 to5 nm. While contacted with it, they are optionally exposed to ultrasonicwaves of from 10 to 50 KHz to enhance the water removal from them.

The base oil for use in the invention of which the essential ingredientis such an oxygen-containing organic compound has a kinematic viscosityat 100° C. of from 5 to 50 mm²/sec, preferably from 7 to 30 mm²/sec,more preferably from 7 to 20 mm²/sec. Base oil having a kinematicviscosity of smaller than 5 mm²/sec could not exhibit good lubricity;and base oil having a kinematic viscosity of larger than 50 mm²/sec willcause power loss and will be useless in practical refrigerators.

The base oil for use in the invention of which the essential ingredientis such an oxygen-containing organic compound has a viscosity index ofat least 60, preferably at least 70, more preferably at least 80. Baseoil having a viscosity index of smaller than 60 is unfavorable, sinceits kinematic viscosity is substantially lowered at high temperaturesand its lubricity will be poor.

The oxygen-containing organic compound for use in the inventionpreferably has at least 2 oxygen atoms in the molecule. Concretely, itincludes polyoxyalkylene glycols, polyvinyl ethers, polyesters andcarbonates, and mixtures of two or more of them are also employableherein. These compounds have good compatibility with carbon dioxiderefrigerant to exhibit good lubricity. They are described in detailhereinunder.

(A-1) Polyoxyalkylene Glycols:

Polyoxyalkylene glycols usable herein are, fore example, compounds of ageneral formula (I):R¹—-[(OR²)_(m)—OR³]_(n)   (I)wherein R¹ represents a hydrogen atom, an alkyl group having from 1 to10 carbon atoms, an acyl group having from 2 to 10 carbon atoms, or analiphatic hydrocarbon group having from 2 to 6 bonding sites and havingfrom 1 to 10 carbon atoms; R² represents an alkylene group having from 2to 4 carbon atoms; R³ represents a hydrogen atom, an alkyl group havingfrom 1 to 10 carbon atoms, or an acyl group having from 2 to 10 carbonatom; n represents an integer of from 1 to 6; and m represents a numberto give a mean value of m×n falling between 6 and 80.

In formula (I), the alkyl group for R¹ and R³may be linear, branched orcyclic. Specific examples of the alkyl group include a methyl group, anethyl group, an n-propyl group, an isopropyl group, all types of butylgroups, all types of pentyl groups, all types of hexyl groups, all typesof heptyl groups, all types of octyl groups, all types of nonyl groups,all types of decyl groups, a cyclopentyl group, a cyclohexyl group, etc.If the number of carbon atoms constituting the alkyl group is largerthan 10, the compatibility of the base oil with refrigerant will be poorand the base oil will be troubled by phase separation from refrigerant.Preferably, the alkyl group has from 1 to 6 carbon atoms.

The alkyl moiety in the acyl group for R¹ and R³ may be linear, branchedor cyclic. As specific examples of the alkyl moiety in the acyl group,referred to are those with from 1 to 9 carbon atoms mentioned above forthe alkyl group. If the number of carbon atoms constituting the acylgroup is larger than 10, the compatibility of the base oil withrefrigerant will be poor and the base oil will be troubled by phaseseparation from refrigerant. Preferably, the acyl group has from 2 to 6carbon atoms.

Where R¹ and R³are both alkyl groups or acyl groups, they may be thesame or different.

Where n in formula (I) is 2 or more, plural R³'s in one molecule of thecompound may be the same or different.

Where R¹ is an aliphatic hydrocarbon group having from 2 to 6 bondingsites and having from 1 to 10 carbon atoms, the aliphatic hydrocarbongroup may be linear or cyclic. Examples of the aliphatic hydrocarbongroup having 2 bonding sites include an ethylene group, a propylenegroup, a butylene group, a pentylene group, a hexylene group, aheptylene group, an octylene group, a nonylene group, a decylene group,a cyclopentylene group, a cyclohexylene group, etc. Examples of thealiphatic hydrocarbon group having from 3 to 6 bonding sites includeresidues to be derived from polyalcohols such as trimethylolpropane,glycerin, pentaerythritol, sorbitol, 1,2,3-trihydroxycyclohexane and1,3,5-trihydroxycyclohexane, by removing the hydroxyl groups from them.

If the number of carbon atoms constituting the aliphatic hydrocarbongroup is larger than 10, the compatibility of the base oil withrefrigerant will be poor and the base oil will be troubled by phaseseparation from refrigerant. Preferably, the aliphatic hydrocarbon grouphas from 2 to 6 carbon atoms.

Preferably, in the invention, at least one of R¹ and R³ is an alkylgroup, more preferably having from 1 to 3 carbon atoms. Especiallypreferred is a methyl group, in view of the viscosity characteristic ofthe base oil. For the same reason, both R¹ and R³are even morepreferably alkyl groups, especially methyl groups.

R² in formula (I) is an alkylene group having from 2 to 4 carbon atoms.The oxyalkylene group for the repetitive units in formula (I) includesan oxyethylene group, an oxypropylene group, and an oxybutylene group.The oxyalkylene groups in one molecule of the compound may be all thesame or different ones. One molecule of the compound may have two ormore different types of oxyalkylene groups. Especially preferred hereinare copolymers having both oxyethylene (EO) and oxypropylene (PO)groups. In those copolymers, it is desirable that the ratio ofEO/(PO+EO) falls between 0.1 and 0.8 in view of the burning load to therefrigerator oil and of the viscosity characteristic of the oil. It isfurther desirable that the ratio of EO/(PO+EO) falls between 0.3 and 0.6in view of the moisture absorption of the oil.

In formula (I), n is an integer of from 1 to 6, and shall be defineddepending on the number of the bonding sites in R¹. For example, whereR¹ is an alkyl group or an acyl group, n is 1; and where it is analiphatic hydrocarbon group having 2, 3, 4, 5 or 6 bonding sites, n is2, 3, 4, 5 or 6, respectively. m is a number to give a mean value of m xn falling between 6 and 80. If the mean value of m x n oversteps thedefined range, the object of the invention could not be attainedsatisfactorily.

Polyoxyalkylene glycols of formula (I) include those having a hydroxylgroup at the terminal. Such hydroxyl-terminated compounds could befavorably used in the invention so far as the terminal hydroxyl contentof the compounds is not larger than 50 mol % of the total terminalcontent thereof. If, however, the terminal hydroxyl content thereof islarger than 50 mol %, the moisture absorption of the compounds willincrease and the viscosity index thereof will decrease. The compoundshaving such a large terminal hydroxyl content are unfavorable to theinvention.

As the polyoxyalkylene glycols for use in the invention, preferred arepolyoxypropylene glycol dimethyl ethers of a general formula:

wherein x represents a number of from 6 to 80; andpolyoxyethylene-polyoxypropylene glycol dimethyl ethers of a generalformula:

wherein a and b each are at least 1, and their total falls between 6 and80,

-   in view of the economical aspect of the compounds and of the effect    thereof.

Also preferred are polyoxypropylene glycol monobutyl ethers of a generalformula:

wherein x represents a number of from 6 to 80, as well aspolyoxypropylene glycol diacetates, in view of the economical aspect ofthe compounds.

Polyalkylene glycols of formula (I) are described in detail in JP-A305893/1990, any of which are employable herein.

Apart from the polyoxyalkylene glycols noted above, further employableherein are polyoxyalkylene glycol derivatives having at least onestructural unit of a general formula (II):

wherein R⁴ to R⁷ each represent a hydrogen atom, a monovalenthydrocarbon group having from 1 to 10 carbon atoms, or a group of ageneral formula (III):

where R⁸ and R⁹ each represent a hydrogen atom, a monovalent hydrocarbongroup having from 1 to 10 carbon atoms, or an alkoxyalkyl group havingfrom 2 to 20 carbon atoms; R¹⁰ represents an alkylene group having from2 to 5 carbon atoms, a substituted alkylene group having an alkyl groupas the substituent and having from 2 to 5 carbon atoms in total, or asubstituted alkylene group having an alkoxyalkyl group as thesubstituent and having from 4 to 10 carbon atoms in total; n representsan integer of from 0 to 20; and R¹¹ represents a monovalent hydrocarbongroup having from 1 to 10 carbon atoms, and at least one of R⁴ to R⁷ isthe group of formula (III).

In formula (II), R⁴ to R⁷ each represent a hydrogen atom, a monovalenthydrocarbon group having from 1 to 10 carbon atoms, or a group offormula (III). The monovalent hydrocarbon group having from 1 to 10carbon atoms is preferably a monovalent hydrocarbon group having at most6 carbon atoms, most preferably an alkyl group having at most 3 carbonatoms.

In formula (III), R⁸ and R⁹ each represent a hydrogen atom, a monovalenthydrocarbon group having from 1 to 10 carbon atoms, or an alkoxyalkylgroup having from 2 to 20 carbon atoms. Preferably, they each are analkyl group having at most 3 carbon atoms, or an alkoxyalkyl grouphaving at most 6 carbon atoms.

R¹⁰ represents an alkylene group having from 2 to 5 carbon atoms, asubstituted alkylene group having an alkyl group as the substituent andhaving from 2 to 5 carbon atoms in total, or a substituted alkylenegroup having an alkoxyalkyl group as the substituent and having from 4to 10 carbon atoms in total. Preferably, it is an ethylene orsubstituted ethylene group having at most 6 carbon atoms.

R¹⁰ represents a monovalent hydrocarbon group having from 1 to 10 carbonatoms. Preferably, it is a hydrocarbon group having at most 6 carbonatoms, more preferably at most 3 carbon atoms.

In formula (II), at least one of R⁴ to R⁷ is a group of formula (III).Preferably, either one of R⁴ and R⁶ is a group of formula (III), and theremaining one of R⁴ and R⁶ and also R⁵ and R⁷ each are a hydrogen atomor a monovalent hydrocarbon group having from 1 to 10 carbon atoms.

The polyoxyalkylene glycol derivatives have at least one structural unitof formula (II). More precisely, they include three types of polymers,which are homopolymers each composed of the structural units of formula(II) of one and the same type, copolymers each composed of at least twodifferent types of the structural units of formula (II), and copolymerseach comprising the structural units of formula (II) and any otherstructural units of, for example, a general formula (IV):

wherein R¹² to R¹⁵ each represent a hydrogen atom or an alkyl grouphaving from 1 to 3 carbon atoms.

Preferred examples of the homopolymers have from 1 to 200 structuralunits A of formula (II) and are terminated with any of a hydroxyl group,an acyloxy group having from 1 to 10 carbon atoms, an alkoxy grouphaving from 1 to 10 carbon atoms, or an aryloxy group.

Preferred examples of the copolymers have two different types ofstructural units A and B each amounting to from 1 to 200 units or havefrom 1 to 200 structural units A of formula (II) and from 1 to 200structural units C of formula (III), and are terminated with any of ahydroxyl group, an acyloxy group having from 1 to 10 carbon atoms, analkoxy group having from 1 to 10 carbon atoms, or an aryloxy group.Various types of these copolymers are employable herein, including, forexample, alternate copolymers, random copolymers or block copolymerscomprising structural units A, structural units B (and/or structuralunits C), as well as graft copolymers with a main chain of structuralunits A as grafted with structural units B.

(A-2) Polyvinyl Ethers:

Polyvinyl ethers for the base oil for use herein may be polyvinyl ethercompounds comprising, for example, structural units of a general formula(V):

wherein R¹⁶, R¹⁷ and R¹⁸ each represent a hydrogen atom, or ahydrocarbon group having from 1 to 8 carbon atoms, and they may be thesame or different; R¹⁹ represents a divalent hydrocarbon group havingfrom 1 to 10 carbon atoms; R²⁰ represents a hydrocarbon group havingfrom 1 to 20 carbon atoms; k represents a number of from 0 to 10 onaverage; R¹⁶ to R²⁰ may be the same or different in different structuralunits; and plural R¹⁹O's, if any, may be the same or different.

Also usable herein are polyvinyl ether compounds of block or randomcopolymers comprising structural units of formula (V) noted above andstructural units of the following general formula (VI):

wherein R²¹ to R²⁴ each represent a hydrogen atom or a hydrocarbon grouphaving from 1 to 20 carbon atoms, and they may be the same or different;and R²¹ to R²⁴ may be the same or different in different structuralunits.

In formula (V), R¹⁶, R¹⁷ and R¹⁸ each represent a hydrogen atom, or ahydrocarbon group having from 1 to 8 carbon atoms, preferably from 14 tocarbon atoms, and they may be the same or different. Concretely, thehydrocarbon group includes an alkyl group including, for example, amethyl group, an ethyl group, an n-propyl group, an isopropyl group, ann-butyl group, an isobutyl group, a sec-butyl group, a tert-butyl group,all types of pentyl groups, all types of hexyl groups, all types ofheptyl groups, and all types of octyl groups; a cycloalkyl groupincluding, for example, a cyclopentyl group, a cyclohexyl group, alltypes of methylcyclohexyl groups, all types of ethylcyclohexyl groups,and all types of dimethylcyclohexyl groups; an aryl group including, forexample, a phenyl group, all types of methylphenyl groups, all types ofethylphenyl groups, and all types of dimethylphenyl groups; and anarylalkyl group including, for example, a benzyl group, all types ofphenylethyl groups, and all types of methylbenzyl groups. Especiallypreferably, R¹⁶, R¹⁷ and R¹⁸ are all hydrogen atoms.

In formula (V), R¹⁹ represents a divalent hydrocarbon group having from1 to 10 carbon atoms, preferably from 2 to 10 carbon atoms. Concretely,the divalent hydrocarbon group having from 1 to 10 carbon atoms includesa divalent aliphatic group including, for example, a methylene group, anethylene group, a phenylethylene group, a 1,2-propylene group, a2-phenyl-1,2-propylene group, a 1,3-propylene group, all types ofbutylene groups, all types of pentylene groups, all types of hexylenegroups, all types of heptylene groups, all types of octylene groups, alltypes of nonylene groups, and all types of decylene groups; an alicyclicgroup with two bonding sites to be derived from an alicyclic hydrocarbonwhich includes, for example, cyclohexane, methylcyclohexane,ethylcyclohexane, dimethylcyclohexane, and propylcyclohexane; a divalentaromatic hydrocarbon group including, for example, all types ofphenylene groups, all types of methylphenylene groups, all types ofethylphenylene groups, all types of dimethylphenylene groups, and alltypes of naphthylene groups; an alkylaromatic group to be derived froman alkylaromatic hydrocarbon such as toluene, xylene or ethylbenzene,and having a monovalent bonding site both in the alkyl moiety and in thearomatic moiety; and an alkylaromatic group to be derived from apolyalkylaromatic hydrocarbon such as xylene or diethylbenzene, andhaving bonding sites in the alkyl moieties. Of those, especiallypreferred are aliphatic groups having from 2 to 4 carbon atoms.

In formula (V), k indicates the number of the repetitive R¹⁹O's, fallingbetween 0 and 10, preferably between 0 and 5 on average. Plural R¹⁹O's,if any, may be the same or different.

In formula (V), R²⁰ represents a hydrocarbon group having from 1 to 20carbon atoms, preferably from 1 to 10 carbon atoms concretely, thehydrocarbon group includes an alkyl group including, for example, amethyl group, an ethyl group, an n-propyl group, an isopropyl group, ann-butyl group, an isobutyl group, a sec-butyl group, a tert-butyl group,all types of pentyl groups, all types of hexyl groups, all types ofheptyl groups, all types of octyl groups, all types of nonyl groups, andall types of decyl groups; a cycloalkyl group including, for example, acyclopentyl group, a cyclohexyl group, all types of methylcyclohexylgroups, all types of ethylcyclohexyl groups, all types ofpropylcyclohexyl groups, and all types of dimethylcyclohexyl groups; anaryl group including, for example, a phenyl group, all types ofmethylphenyl groups, all types of ethylphenyl groups, all types ofdimethylphenyl groups, all types of propylphenyl groups, all types ortrimethylphenyl groups, all types of butylphenyl groups, and all typesof naphthyl groups; and an arylalkyl group including, for example, abenzyl group, all types of phenylethyl groups, all types of methylbenzylgroups, all types of phenylpropyl groups, and all types of phenylbutylgroups.

R¹⁶ to R²⁰ may be the same or different in different structural units ofthe polyvinyl ether compounds (1) of formula (V), preferred are those inwhich the ratio by mol of carbon/oxygen falls between 4.2 and 7.0. Ifthe molar ratio is smaller than 4.2, the moisture absorption of thecompounds will be high; but if larger than 7.0, the compatibility of thecompounds with refrigerant will be poor.

In formula (VI), R²¹ to R²⁴ each represent a hydrogen atom or ahydrocarbon group having from 1 to 20 carbon atoms, and they may be thesame or different. For examples of the hydrocarbon group having from 1to 20 carbon atoms, referred to are the same as those mentionedhereinabove for R²⁰ in formula (V). R²¹ to R²⁴ may be the same ordifferent in different structural units.

Of the polyvinyl ether compounds (2) of block or random copolymerscomprising structural units of formula (V) and structural units offormula (VI), preferred are those in which the ratio by mol ofcarbon/oxygen falls between 4.2 and. 7.0. If the molar ratio is smallerthan 4.2, the moisture absorption of the compounds will be high; but iflarger than 7.0, the compatibility of the compounds with refrigerantwill be poor.

In the invention, further employable are mixtures of the polyvinyl ethercompound (1) and the polyvinyl ether compound (2).

Those polyvinyl ether compounds (1) and (2) may be produced throughpolymerization of vinyl ether monomers corresponding thereto, or throughcopolymerization of hydrocarbon monomers having olefinic double bondsand corresponding thereto with vinyl ether monomers also correspondingthereto.

Also preferably, the polyvinyl ether compounds for use in the inventionare specifically terminated in the manner mentioned below. As onepreferred example of the terminal structure of the compounds, one end ofthe molecule is terminated with a group of a general formula (VII) or(VIII):

wherein R²⁵, R²⁶ and R²⁷ each represent a hydrogen atom or a hydrocarbongroup having from 1 to 8 carbon atoms, and they may be the same ordifferent; R³⁰, R³¹, R³² and R³³ each represent a hydrogen atom or ahydrocarbon group having from 1 to 20 carbon atoms, and they may be thesame or different; R²⁸ represents a divalent hydrocarbon group havingfrom 1 to 10 carbon atoms; R²⁹ represents a hydrocarbon group havingfrom 1 to 20 carbon atoms; p represents a number of from 0 to 10 onaverage; and plural R²⁸O's, if any, may be the same or different, andthe other end thereof is terminated with a group of a general formula(IX) or (X):

wherein R³⁴, R³⁵ and R³⁶ each represent a hydrogen atom or a hydrocarbongroup having from 1 to 8 carbon atoms, and they may be the same ordifferent; R³⁹, R⁴⁰, R⁴¹ and R⁴² each represent a hydrogen atom or ahydrocarbon group having from 1 to 20 carbon atoms, and they may be thesame or different; R³⁷ represents a divalent hydrocarbon group havingfrom 1 to 10 carbon atoms; R³⁸ represents a hydrocarbon group havingfrom 1 to 20 carbon atoms; q represents a number of from 0 to 10 onaverage; and plural R³⁷O's, if any, may be the same or different.

As another preferred example of the terminal structure of the compounds,one end of the molecule is terminated with a group of formula (VII) or(VIII) as above and the other end thereof is terminated with a group ofa general formula (XI):

wherein R⁴³, R⁴⁴ and R⁴⁵ each represent a hydrogen atom or a hydrocarbongroup having from 1 to 8 carbon atoms, and they may be the same ordifferent.

Of those polyvinyl ether compounds, especially preferred herein are thefollowing:

(1) Compounds comprising structural units of formula (V) and terminatedwith a group of formula (VII) or (VIII) at one end and with a group offormula (IX) or (X) at the other end, in which R¹⁶, R¹⁷ and R¹⁸ in theunit of formula (V) are all hydrogen atoms, k is a number of from 0 to4, R¹⁹ is a divalent hydrocarbon group having from 2 to 4 carbon atoms,and R²⁰ is a hydrocarbon group having from 1 to 20 carbon atoms.

(2) Compounds composed of structural units of formula (V) only andterminated with a group of formula (VII) at one end and with a group offormula (IX) at the other end, in which R¹⁶, R¹⁷ and R⁸ in the unit offormula (V) are all hydrogen atoms, k is a number of from 0 to 4, R⁹ isa divalent hydrocarbon group having from 2 to 4 carbon atoms, and R²⁰ isa hydrocarbon group having from 1 to 20 carbon atoms.

(3) Compounds comprising structural units of formula (V) and terminatedwith a group of formula (VII) or (VIII) at one end and with a group offormula (XI) at the other end, in which R¹⁶, R¹⁷ and R¹⁸ in the unit offormula (V) are all hydrogen atoms, k is a number of from 0 to 4, R¹⁹ isa divalent hydrocarbon group having from 2 to 4 carbon atoms, and R²⁰ isa hydrocarbon group having from 1 to 20 carbon atoms.

(4) Compounds composed of structural units of formula (V) only andterminated with a group of formula (VII) at one end and with a group offormula (X) at the other end, in which R¹⁶, R¹⁷ and R¹⁸ in the unit offormula (V) are all hydrogen atoms, k is a number of from 0 to 4, R¹⁹ isa divalent hydrocarbon group having from 2 to 4 carbon atoms, R²⁰ is adivalent hydrocarbon group having from 1 to 20 carbon atoms, and R²⁰ isa hydrocarbon group having from 1 to 20 carbon atoms.

In the invention, also usable are polyvinyl ether compounds comprisingstructural units of formula (V) and terminated with a group of formula(VII) noted above at one end and with a group of the following generalformula (XII) at the other end:

wherein R⁴⁶, R⁴⁷ and R⁴⁸ each represent a hydrogen atom or a hydrocarbongroup having from 1 to 8 carbon atoms, and they may be the same ordifferent; R⁴⁹ and R⁵¹ each represent a divalent hydrocarbon grouphaving from 2 to 10 carbon atoms, and they may be the same or different;R⁵⁰ and R⁵² each represent a hydrocarbon group having from 1 to 10carbon atoms, and they may be the same or different; c and d eachrepresent a number of from 0 to 10 on average, and they may be the sameor different; and plural R⁴⁹O's, if any, may be the same or different,and plural R⁵¹O's, if any, may also be the same or different.

Further usable herein are polyvinyl ether compounds of homopolymers orcopolymers of alkyl vinyl ethers, which comprise structural units of ageneral formula (XIII) or (XIV):

wherein R⁵³ represents a hydrocarbon group having from 1 to 8 carbonatoms,

-   and have a weight-average molecular weight of from 300 to 5,000, and    of which one end is terminated with a group of a general    formula (XV) or (XVI):    wherein R⁵⁴ represents an alkyl group having from 1 to 3 carbon    atoms; and R⁵⁵ represents a hydrocarbon group having from 1 to 8    carbon atoms.

Some polyvinyl ethers are described in detail in JP-A 128578/1994, JP-A234814/1994, JP-A 234815/1994 and JP-A 113196/1996, all of which areusable herein.

(A-3) Polyesters:

Polyesters for use in the invention include <1> polyalcohol esters, and<2> polycarboxylates.

<1> Polyalcohol Esters:

The polyalcohol esters are esters of polyalcohols with monocarboxylicacids, encompassing complex esters of polyalcohols with mixed fattyacids of monocarboxylic acids and polycarboxylic acids.

Typical examples of the polyalcohol esters are esters of aliphaticpolyalcohols with linear or branched fatty acids.

The aliphatic polyalcohols for the esters include, for example, ethyleneglycol, propylene glycol, butylene glycol, neopentyl glycol,trimethylolethane, ditrimethylolethane, trimethylolpropane,ditrimethylolpropane, glycerin, pentaerythritol, dipentaerythritol,tripentaerythritol, and sorbitol.

The fatty acids may have from 3 to 12 carbon atoms. As preferredexamples of the fatty acids, mentioned are propionic acid, butyric acid,pivalic acid, valeric acid, caproic acid, heptanoic acid, octanoic acid,nonanoic acid, decanoic acid, dodecanoic acid, isovaleric acid,neopentanoic acid, 2-methylbutyric acid, 2-ethylbutyric acid,2-methylhexanoic acid, 2-ethylhexanoicacid, isooctanoicacid,isononanoicacid, isodecanoic acid, 2,2-dimethyloctanoic acid,2-butyloctanoic acid, and 3,5,5-trimethylhexanoic acid. Semiesters ofaliphatic polyalcohols with linear or branched fatty acids are alsoemployable herein.

As the esters of aliphatic polyalcohols with linear or branched fattyacids, especially preferred are esters of pentaerythritol,dipentaerythritol or tripentaerythritol with fatty acids having from 5to 12 carbon atoms, more preferably from 5 to 9 carbon atoms, such asvaleric acid, hexanoic acid, heptanoic acid, 2-methylhexanoic acid,2-ethylhexanoic acid, isooctanoic acid, isononanoic acid, isodecanoicacid, 2,2-dimethyloctanoic acid, 2-butyloctanoic acid, and3,5,5-trimethylhexanoic acid.

Also usable herein are complex esters composed of semiesters ofaliphatic polyalcohols with linear or branched C3-9 fatty acids, andaliphatic dibasic acids or aromatic dibasic acids. For such complexesters, preferred are C5-7 fatty acids, more preferably C5-6 fattyacids. The fatty acids of that type include valeric acid, hexanoic acid,isovaleric acid, 2-methylbutyric acid, 2-ethylbutyric acid, and theirmixtures. Especially preferred are mixtures of a C5 fatty acid and a C6fatty acid as mixed in a ratio by weight falling between 10/90 and90/10. Along with those fatty acids, used are aliphatic dibasic acidsfor esterification with polyalcohols. The aliphatic dibasic acidsinclude succinic acid, adipic acid, pimelic acid, suberic acid, azelaicacid, sebacic acid, undecane-diacid, dodecane-diacid, tridecane-diacid,and docosane-diacid. Also usable are aromatic dibasic acids, includingphthalic acid and isophthalic acid. The esterification for preparing thecomplex esters comprises reacting a polyalcohol with a dibasic acid in apredetermined ratio to give a semiester followed by reacting theresulting semiester with a fatty acid, or the order of reacting apolyalcohol with a dibasic acid and a fatty acid may be reversed, or adibasic acid and a fatty acid may be mixed and reacted both at a timewith a polyalcohol for the intended esterification.

Also preferred for use herein are polyalcohol esters to be prepared byreacting an acid fluoride of a general formula (XVII):

wherein R⁵⁶ to R⁵⁸ each represent an alkyl group having from 1 to 13carbon atoms, provided that the alkyl group having 4 or more carbonatoms shall have at least one branch and that the total of the carbonatoms constituting R⁵⁶ to R⁵⁸ shall fall between 3 and 23, with apolyalcohol, as their moisture absorption to saturation is low (see JP-A157219/1997).<2> Polycarboxylates:

Polycarboxylates are usable herein, which are esters of polycarboxylicacids with a monoalcohol or its derivative (monoalcohols) and encompasscomplex esters of polycarboxylic acids with mixed alcohols ofmonoalcohols and polyalcohols. The polycarboxylates of the type includealiphatic or aromatic polycarboxylates, and alicyclic polycarboxylates.

Examples of the aliphatic or aromatic polycarboxylates are dialkylesters aliphatic or aromatic dicarboxylic acids (having from 16 to 22carbon atoms).

The aliphatic dicarboxylic acids include, for example, succinic acid,glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid,sebacic acid, undecane-diacid, dodecane-diacid, tridecane-diacid, anddocosane-diacid; and the aromatic dicarboxylic acids include, forexample, phthalic acid, and isophthalic acid. The alcohol components forthe esters may be of C5-8 alcohols, including, for example, amylalcohol, hexyl alcohol, heptyl alcohol, and octyl alcohol. Preferredexamples of the esters are dioctyl adipate, diisoheptyl adipate, dihexylsebacate, diheptyl succinate, dioctyl phthalate, diisoheptyl phthalate,and diisoamyl phthalate.

Aliphatic or aromatic, tri- or higher poly-carboxylates are also usableherein, for which tri- or higher polycarboxylic acids include, forexample, aliphatic polycarboxylic acids such as1,2,3,4-butane-tetracarboxylic acid, and aromatic polycarboxylic acidssuch as trimellitic acid, and pyromellitic acid. The alcohol componentsfor the esters may bemonoalcohols with a linear or branched C3-12 alkylgroup, or polyalkylene glycol monoalcohols of a formula, H—(R′O)_(n)—Rin which R′ indicates a C2-8 alkylene group, R indicates a C1-10 alkylgroup, and n is an integer of from 1 to 10. Herein usable arepolycarboxylates to be prepared by esterifying the polycarboxylic acidsmentioned above with alcohols such as those also mentioned above, aswell as complex esters to be prepared by reacting those polycarboxylicacids with those alcohols and further with other polyalcohols such asethylene glycol or propylene glycol for esterification.

Alicyclic polycarboxylates are also usable herein, which may berepresented by the following general formula (XVIII):

wherein A represents a cyclohexane ring or a cyclohexene ring; R⁵⁹represents a hydrogen atom or a methyl group; X represents a hydrogenatom or COOR⁶²; Y represents a hydrogen atom or COOR⁶³; and R⁶⁰ to R⁶¹each indicate a C3-18 alkyl group or a C3-10 cycloalkyl group, and theymay be the same or different.

These esters may be prepared by esterifying a predetermined acidcomponent with a predetermined alcohol component in any ordinary manner,but preferably in an inert gas atmosphere of nitrogen gas or the like,in the presence or absence of an esterification catalyst, and under heatwith stirring the components.

The acid component includes cycloalkane-polycarboxylic acids,cycloalkene-polycarboxylic acids and their anhydrides. One or more ofthose compounds may be used either singly or as combined, for the acidcomponent. Concretely mentioned are 1,2-cyclohexane-dicarboxylic acid,4-cyclohexene-1,2-dicarboxylic acid, 1-cyclohexene-1,2-dicarboxylicacid, 1,3-cyclohexane-dicarboxylic acid, 1,4-cyclohexane-dicarboxylicacid, 3-methyl-1,2-cyclohexane-dicarboxylic acid,4-methyl-1,2-cyclohexane-dicarboxylic acid,3-methyl-4-cyclohexene-1,2-dicarboxylic acid,4-methyl-4-cyclohexene-1,2-dicarboxylic acid,1,2,4-cyclohexane-tricarboxylic acid, 1,3,5-cyclohexane-tricarboxylicacid, 1,2,4,5-cyclohexane-tetracarboxylic acid, and their anhydrides. Ofthose, preferred are 1,2-cyclohexane-dicarboxylic acid,3-methyl-1,2-cyclohexane-dicarboxylic acid,4-methyl-1,2-cyclohexane-dicarboxylic acid,4-cyclohexene-1,2-dicarboxylic acid,3-methyl-4-cyclohexene-1,2-dicarboxylic acid,4-methyl-4-cyclohexene-1,2-dicarboxylic acid, and their anhydrides.

The alcohol component includes linear or branched aliphatic C3-18alcohols, and alicyclic C3-10 alcohols.

Specific examples of the linear aliphatic alcohols are n-propyl alcohol,n-butanol, n-pentanol, n-hexanol, n-heptanol, n-octanol, n-nonanol,n-decanol, n-undecanol, n-dodecanol, n-tetradecanol, n-hexadecanol,n-octadecanol.

Specific examples of the branched aliphatic alcohols are isopropanol,isobutanol, sec-butanol, isopentanol, isohexanol, 2-methylhexanol,2-methylheptanol, isoheptanol, 2-ethylhexanol, 2-octanol, isooctanol,3,5,5-trimethylhexanol, isodecanol, isoundecanol, isotridecanol,isotetradecanol, isohexadecanol, isooctadecanol,2,6-dimethyl-4-heptanol.

Specific examples of the alicyclic alcohols are cyclohexanol,methylcyclohexanol, dimethylcyclohexanol.

Of those alicyclic polycarboxylates to be prepared from suchpolycarboxylic acids and alcohols, especially preferred are diisobutyl1,2-cyclohexane-dicarboxylate, dicyclohexyl1,2-cyclohexane-dicarboxylate, diisoheptyl1,2-cyclohexane-dicarboxylate,di(2-ethylhexyl)1,2-cyclohexane-dicarboxylate,di(3,5,5-trimethylhexyl) 1,2-cyclohexane-dicarboxylate,di(2,6-dimethyl-4-heptyl) 1,2-cyclohexane-dicarboxylate, diisodecyl1,2-cyclohexane-dicarboxylate, diisoundecyl1,2-cyclohexane-dicarboxylate, dicyclohexyl4-cyclohexene-1,2-dicarboxylate, diisoheptyl4-cyclohexene-1,2-dicarboxylate, di(2-ethylhexyl)4-cyclohexene-1,2-dicarboxylate, di(3,5,5-trimethylhexyl)4-cyclohexene-1,2-dicarboxylate, di(3,5,5-trimethylhexyl)3-methyl-1,2-cyclohexane-dicarboxylate, di(3,5,5-trimethylhexyl)4-methyl-1,2-cyclohexane-dicarboxylate, di(3,5,5-trimethylhexyl)3-methyl-4-cyclohexene-1,2-dicarboxylate, di(3,5,5-trimethylhexyl)4-methyl-4-cyclohexene-1,2-dicarboxylate, andtetra(3,5,5-trimethylhexyl) 1,2,4,5-cyclohexane-tetracarboxylate.

The alicyclic polycarboxylates may be mixed with any other esters(hereinafter these are referred to as additional esters), with whichtheir physical balance including volume-specific resistivity andviscosity could be improved. The additional esters include adipates,azelates, sebacates, phthalates, trimellitates, and polyalcohol esters.The polyalcohol component for the polyalcohol esters includes, forexample, neopentyl glycol, trimethylolpropane, pentaerythritol,dipentaerythritol. The acid component for them includes, for example,isobutyric acid, 2-ethylbutyric acid, isovaleric acid, pivalic acid,cyclohexanecarboxylic acid, 2-methylpentanoic acid, 2-ethylpentanoicacid, 2-methylhexanoic acid, 2-ethylhexanoic acid,3,5,5-trimethylhexanoic acid.

Apart from <1> and <2> mentioned above, further employable herein arediesters to be prepared by esterifying monoalcohol-alkylene oxideadducts with aliphatic dicarboxylic acids such as adipic acid, pimelicacid, suberic acid, azelaic acid, sebacic acid, undecane-diacid,dodecane-diacid, or docosane-diacid, or with aromatic dicarboxylic acidssuch as phthalic acid. Still further employable are esters to beprepared by esterifying polyalcohol-alkylene oxide (1 to 10 mols)adducts in which the polyalcohol maybe, for example, glycerin ortrimethylolpropane, with C3-12 fatty acids such as propionic acid,butyric acid, valeric acid, hexanoic acid, heptanoic acid, octanoicacid, nonanoic acid, decanoic acid, dodecanoic acid, 2-methylhexanoicacid, 2-ethylhexanoic acid, isooctanoic acid, isononanoic acid,isodecanoic acid, 2,2-dimethyloctanoic acid, or 2-butyloctanoic acid.

<3> Other Polyesters:

Still other polyesters employable herein are fumarate oligomers andhydroxypivalates.

The fumarate oligomers may be homopolymers of fumarates or copolymers offumarates with unsaturated aliphatic hydrocarbons, and may berepresented by a general formula (XIX):

wherein R⁶⁴ represents an alkylene group, a substituted alkylene group,or an alkylene oxide group; R⁶⁵ and R⁶⁶ each represent a C1-9 alkylgroup, an allyl group, or a terminal-substituted or unsubstitutedpolyalkylene oxide group, and these may be the same or different; e is 0or an integer of at least one, f is an integer of at least one; and R⁷⁰is not larger than 50 mol % of the oligomer.

Concretely mentioned are diethyl fumarate oligomers and dibutyl fumarateoligomers.

In formula (XIX), both terminals are of residues of the polymerizationinitiator used in polymerization to give the oligomers, and are notshown.

Also mentioned are alkyl fumarate copolymers comprising from 1 to 50 mol% of structural units of a formula (XX):

and from 50 to 99 mol % of structural units of a general formula (XXI):

wherein R⁶⁷ and R⁶⁸ each represent a C3-8 alkyl group, and they may bethe same or different.

The hydroxypivalates may be represented by a general formula (XXII):

wherein R69 and R⁷⁰ each represent a C2-10 alkyl group, and g is aninteger of from 1 to 5.(A-4) Carbonates:

Carbonates for use herein include compounds of a general formula(XXIII):

wherein R⁷¹ represents a C2-10 alkyl group; R⁷²represents a C2-10alkylene or cycloalkylene group; and h is an integer of from 1 to 4, andcompounds of a general formula (XXIV):

wherein R⁷³ represents a hydroxyl group-having C2-6 polyalcohol residue;R⁷⁴ represents a C2-10 alkyl group; and i is an integer of from 2 to 6.

Those carbonates may be prepared by interesterifying dimethyl carbonatewith alcohols in the presence of a basic catalyst.

Also usable herein are compounds of a general formula (XXV):

wherein R⁵ represents a C1-10 alkyl group; R⁶ represents a C2-10 alkylgroup; j is an integer of from 2 to 10; 1 is an integer of from 2 to100; and —BO— represents —CH₂—CH(CH₃)—O— or —CH₂—CH₂—O—.

The carbonates may be prepared, for example, by reacting carbonic acidwith alkylene oxides. In those, the alkylene oxide moiety added to thecarbonate moiety may be from a single ethylene oxide or a singlepropylene oxide or may also be from their mixture.

Of the oxygen-containing compounds of (A-1) to (A-4), especiallypreferred are those of (A-1) and (A-2) as stable.

The base oil for use in the invention may comprise one and the same typeof or two or more different types of the oxygen-containing compounds of(A-1) to (A-4) either singly or as combined. For example, it maycomprise one compound (A-1), or may comprise two or more of the sametype of the compounds (A-1), or may even comprise two or more ofdifferent types of compounds (A-1) and (A-2), either singly or ascombined.

The base oil shall comprise any of the compounds (A-1) to (A-4) as theessential component, but, if desired, it may optionally contain a minoramount of any other compounds of, for example, mineral oils,hydrocarbons such as poly-α-olefins, monoesters, etc.

Preferably, the refrigerator oil composition of the invention containsan acid scavenger. Combined with an acid scavenger, theoxygen-containing compound having a reduced water content in thecomposition is more effective for protecting refrigerator oil from beingdegraded and for protecting metallic machine parts from being corroded.

The acid scavenger usable herein includes, for example, epoxy compoundssuch as phenyl glycidyl ethers, alkyl glycidyl ethers, alkylene glycolglycidyl ethers, cyclohexene oxides, α-olefin oxides, and epoxidatedsoybean oils. Of those, preferred are phenyl glycidyl ethers, alkylglycidyl ethers, alkylene glycol glycidyl ethers, cyclohexene oxides andα-olefin oxides, in view of their compatibility with base oil.

The alkyl group in the alkyl glycidyl ethers and the alkylene group inthe alkylene glycol glycidyl ethers may be branched, and the number ofcarbon atoms constituting the group falls generally between 3 and 30,preferably between 4 and 24, more preferably between 6 and 16. Thenumber of all carbon atoms constituting the α-olefin oxide fallsgenerally between 4 and 30, preferably between 4 and 24, more preferablybetween 6 and 16.

The refrigerator oil composition of the invention may contain one ormore of those acid scavengers either singly or as combined. The acidscavenger content of the refrigerator oil composition preferably fallsbetween 0.005 and 5% by weight relative to the base oil. If the contentis smaller than 0.005% by weight, the acid scavenger could not exhibitits acid-scavenging capabilities. On the other hand, if the content islarger than 5% by weight, the acid scavenger will cause sludge formationin the refrigerator oil composition.

If desired, the refrigerator oil composition of the invention mayfurther contain any known additives, for example, an extreme pressureagent such as phosphates (e.g., tricresyl phosphate, TCP) orphosphites(e.g., trisnonylphneylphosphite, dioleyl hydrogenphosphite); aphenol-type antioxidant such as 2,6-di-tert-butyl-4-methylphenol or anamine-type antioxidant such as phenyl-α-naphthylamine; a copperinactivator such as benzotriazole,N-[N,N′-di(C3-12)alkylaminomethyl]tolutriazoles; and a defoaming agentsuch as silicone oil or fluorosilicone oil. Also if desired, it maycontain any other additives of, for example, load reduction improver,chlorine scavenger, detergent dispersant, viscosity index improver, oil,rust inhibitor, corrosion inhibitor and pour point depressant. Theamount of the additives in the composition of the invention generallyfalls between 0.01 and 10% by weight.

The carbon dioxide refrigerant referred to herein indicates any andevery one comprising carbon dioxide as the essential component, and itmay contain any additional refrigerant components of, for example,hydrocarbons such as propane or isobutane; ammonia; Flons such ashydrofluorocarbons and fluorocarbons, typically1,1,1,2-tetrafluoroethane (R-134a). Even though the refrigerant containsany such additional components but so far as it comprises carbon dioxideas the essential component, the refrigerator oil composition of theinvention is effective thereto and produces good results when appliedthereto.

In the method of using the refrigerator oil composition of the inventionfor lubricating refrigerators, it is desirable that the ratio of thecarbon dioxide-based refrigerant to be combined with the refrigeratoroil composition to the refrigerator oil, refrigerant/refrigerator oilcomposition, falls between 99/1 and 10/90 by weight. If the amount ofthe refrigerant is smaller than the defined range, the refrigeratingcapabilities of the refrigerator oil composition combined with therefrigerant will be poor. If, on the other hand, the amount of therefrigerant is larger than that range, the lubricating capabilities ofthe refrigerator oil composition will be poor. Anyhow, the amount of therefrigerant overstepping the defined range is unfavorable. For thesereasons, the ratio by weight of refrigerant/refrigerator oil compositionmore preferably falls between 95/5 and 30/70.

The refrigerator oil composition of the invention is applicable tovarious types of refrigerators, and is especially suitable to thecompression refrigeration cycle for compression refrigerators. Inparticular, the refrigerator oil composition of the invention exhibitsits effects more effectively, when applied to compression refrigerationcycles equipped with an oil separator and/or a hot gas line, such asthose illustrated in FIG. 1 to 3 attached hereto. As a rule, acompression refrigeration cycle comprises compressor-condenser-expansionvalve-evaporator. The lubricating oil for refrigerators generally hasgood compatibility with the refrigerant for them. However, when a carbondioxide-based refrigerant is used in the refrigeration cycle as aboveand when the refrigerator is lubricated with ordinary refrigerator oil,the ability of the oil to prevent machine parts from being worn awaywill be often poor and the stability of the oil will be also poor. As aresult, it is often impossible to use the refrigerator oil for a longperiod of time. In particular, this tendency is noticeable when therefrigerator oil is used in refrigeration cycles for electricrefrigerators or small-sized air conditioners in which a capillary tubeis used as the expansion valve. In that situation, the refrigerator oilcomposition of the invention is still effective even in driving acompression refrigeration cycle equipped with an oil separator and/or ahot gas line, for which is used a carbon dioxide-based refrigerant.

The invention is described in more detail with reference to thefollowing Examples.

EXAMPLES 1 TO 8, AND COMPARATIVE EXAMPLES 1 TO 9

Oxygen-containing organic compounds shown in Table 1 were tested for thestability in carbon dioxide refrigerant. In the stability test, thetotal acid value increase was measured. To every sample, added was 1.2%by weight of an acid scavenger, α-olefin oxide. The test result is givenin Table 2.

In Table 1, polypropylene glycol polycarbonates of sample Nos. XVII andXVIII were prepared according to the method of Examples 3 and 2,respectively, of JP-A 8725/1992.

The details of the stability test are as follows:

[Stability Test]

40 g of an oil sample, 40 g of carbon dioxide refrigerant gas, and ametal catalyst of copper, aluminium and iron were put into a 120 mlautoclave, to which was added water to make the system have a watercontent of 2000 ppm. The autoclave was sealed, and kept at 175° C. for10 days. After thus left, the oil sample was analyzed. During theexperiment, the inner pressure was kept at 15 MPa. TABLE 1 KinematicViscosity Water at 100° C. Viscosity Content No. Sample (mm²/s) Index(ppm) I PAG-A*1 10.9 212 500 II PAG-A dehydrate — — 150 III PAG-Ahydrate — — 3000 IV PAG-B*2 dehydrate 20.5 218 220 V PAG-B hydrate — —1900 VI PAG-C*3 dehydrate  9.7 185 250 VII PAG-C hydrate — — 3500 VIIIPAG-D*4 dehydrate 14.7  94 170 IX PAG-D hydrate — — 2600 X PVE*5dehydrate  9.8  86 70 XI PVE hydrate — — 5000 XII POE-A*6 14.7  94 550XIII POE-A dehydrate — — 80 XIV POE-A hydrate — — 2200 XV POE-B*7dehydrate  9.1  92 240 XVI POE-B hydrate — — 2500 XVII PC-A*8 dehydrate13.0 120 200 XVIII PC-B*9 dehydrate 10.2 118 150Every sample in the table contains 1.2% by weight of an acid scavenger,α-olefin oxide.*1polyoxypropylene glycol dimethyl ether*2polyoxyethylene-oxypropylene glycol dimethyl ether [EO:PO = 2:8 bymol]*3polyoxypropylene glycol monomethyl ether*4polyoxyethylene-oxypropylene glycol mono-n-butyl ether [EO:PO = 1:9 bymol]*5polyethyl vinyl ether/polyisobutyl vinyl ether [9:1 by mol] copolymer*6pentaerythritol/2-ethylhexanoic acid + 3,5,5-trimethylhexanoic acid[2:8 by mol]*7pentaerythritol/3,5,5-trimethylhexanoic acid*8polypropylene glycol polycarbonate*9polypropylene glycol polycarbonate

TABLE 2 Sample Stability Test Total Acid Value No. Appearance of OilPrecipitate Metal Catalyst Increase (mgKOH/g) Comp. Ex. 1 I good nobrown spots found (aluminium plate) 0.05 Ex. 1 II good no no change 0.01Comp. Ex. 2 III pale yellow no brown spots found (aluminium plate) 0.10Ex. 2 IV good no no change 0.01 Comp. Ex. 3 V good no brown spots found(aluminium plate) 0.12 Ex. 3 VI good no no change 0.02 Comp. Ex. 4 VIIpale yellow no copper plating (small) 0.16 Ex. 4 VIII good no no change0.02 Comp. Ex. 5 IX pale yellow no brown spots found (aluminium plate)0.18 Ex. 5 X good no no change 0.01 or less Comp. Ex. 6 XI good no brownspots found (aluminium plate) 0.08 Comp. Ex. 7 XII pale yellow no brownspots found (aluminium plate) 0.06 Ex. 6 XIII good no no change 0.03Comp. Ex. 8 XIV pale yellowish brown yes copper plating (medium) 1.45Ex. 7 XV good no no change 0.02 Comp. Ex. 9 XVI pale yellowish brown yescopper plating (great) 2.37 Ex. 8 XVII good no no change 0.03 Ex. 9 XVIIgood no no change 0.02

INDUSTRIAL APPLICABILITY

The refrigerator oil composition for carbon dioxide refrigerant of theinvention is stable even in a supercritical carbon dioxide atmosphere.Its total acid value increase is small, and it degrades little. Inaddition, it is effective for preventing metal corrosion and copperplating.

1. A refrigerator oil composition for carbon dioxide refrigerant, ofwhich the base oil comprises polyoxyalkylene glycols and/or polyvinylethers as the essential component and has a kinematic viscosity at 100°C. of from 5 to 50 mm 2/sec, a viscosity index of at least 60, and awater content of at most 300 ppm.
 2. The refrigerator oil compositionfor carbon dioxide refrigerant as claimed in claim 1, of which the watercontent of the base oil is at most 250 ppm.
 3. (canceled)
 4. Therefrigerator oil composition for carbon dioxide refrigerant as claimedin claim 1, wherein the polyoxyalkylene glycols are represented by thefollowing general formula (I) and the polyvinyl ethers are by thefollowing formula (V):R¹—[(OR²)_(m)—OR³]_(n)   (I) where R¹ represents a hydrogen atom, analkyl group having from 1 to 10 carbon atoms, an acyl group having from2 to 10 carbon atoms, or an aliphatic hydrocarbon group having from 2 to6 bonding sites and having from 1 to 10 carbon atoms; R² represents analkylene group having from 2 to 4 carbon atoms; R³ represents a hydrogenatom, an alkyl group having from 1 to 10 carbon atoms, or an acyl grouphaving from 2 to 10 carbon atom; n represents an integer of from 1 to 6;and m represents a number to give a mean value of m×n falling between 6and 80;

where R¹⁶, R¹⁷ and R¹⁸ each represent a hydrogen atom, or a hydrocarbongroup having from 1 to 8 carbon atoms, and they may be the same ordifferent; R¹⁹ represents a divalent hydrocarbon group having from 1 to10 carbon atoms; R²⁰ represents a hydrocarbon group having from 1 to 20carbon atoms; k represents a number of from 0 to 10 on average; R¹⁶ toR²⁰ may be the same or different in different structural units; andplural R¹⁹O's, if any, may be the same or different. 5-7. (canceled)